1. Field of the Invention
This invention relates to an apparatus and method for coating implantable devices such as stents.
2. Description of the Background
Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location. Mechanical intervention via stents has reduced the rate of restenosis; restenosis, however, is still a significant clinical problem. Accordingly, stents have been modified to perform not only as a mechanical scaffolding, but also to provide biological therapy.
Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
A common method of medicating a stent is by depositing a polymeric coating, impregnated with the therapeutic substance, on the surface of the stent. A polymer dissolved in a solvent is applied to the stent. A therapeutic substance can be dissolved or dispersed in the composition. The solvent is allowed to evaporate to form the coating. The application of the composition can be performed by spraying the composition on the stent or immersing the stent in the composition.
The solvents employed with the composition can be categorized as having a high vapor pressure or low vapor pressure. Non-volatile solvents evaporate very slowly from the composition causing coating defects such as inconsistency in the coating thickness and formation of xe2x80x9ccob websxe2x80x9d or xe2x80x9cpool websxe2x80x9d between the stent struts. A solution to this problem is to coat the stent at elevated temperatures to increase the evaporation rate of the solvent. However, not all drugs are stable at elevated temperatures. Volatile solvents have the tendency to evaporate very quickly from the composition resulting in a coating which has a powdered consistency and adheres poorly to the surface of the stent. Accordingly, what is needed is an apparatus and process for coating stents that does not suffer from the aforementioned drawbacks.
accordance with one aspect of the invention, a method of forming a coating for an implantable medical device, such as a stent, is provided. The method comprises applying a composition to the stent in an environment having a pressure other than ambient pressure. For compositions including a non-volatile solvent, the pressure can be less that 760 torr; for compositions including a volatile solvent, the pressure can be greater than 760 torr. The composition can include a polymer, such as an ethylene vinyl alcohol copolymer dissolved in a solvent, such as dimethylacetamide. Optionally, a therapeutic substance can be added to the composition, such as actinomycin D, paclitaxel, docetaxel, or rapamycin. In accordance to one embodiment, the composition can be applied by spraying the composition on the stent. During the act of applying, the stent can be rotated and/or moved in a linear direction along the longitudinal axis of the stent. The stent can be a radially expandable stent, such as a balloon expandable or self-expandable type.
In accordance with another aspect of the invention, a method of forming a coating for a stent is provided, comprising positioning a stent in a chamber; applying a fluid to the stent; and adjusting the pressure of the chamber to increase or decrease the evaporation rate of the fluid.
In accordance with another aspect of the invention, an apparatus for coating implantable medical devices such as stents is provided. The apparatus includes a chamber for housing a stent and a pressure controller for adjusting the pressure of the chamber during the coating process to a pressure below or above 760 torr. In one embodiment, an applicator can be provided for spraying a composition at the stent. A support assembly holds the stents in the chamber and can be connected to a motor for providing rotational and/or translational motion to the stent. A temperature controller can also be provided for adjusting the temperature of the chamber.